Should Streams be Managed as Drainage or Habitat Networks?

by Michael M. Pollock, NOAA Fisheries

Many hydraulic engineers and fluvial geomorphologists have historically viewed stream systems as drainage networks that if functioning properly, should efficiently route water and sediment downstream. In this view, natural instream obstructions such as wood, rocks or beaver dams are not desirable because they may create unpredictable changes in channel morphology that would impact the movement of sediment and water and may affect stream stability. This view has extended to the science of stream restoration and the regulatory environment that guides restoration projects, where channels must often be engineered to remain vertically and laterally stable and to move sediment and water at the rate designed over the life of the project. If instream structures are included in a restoration project, they must often be designed to be stable and to not substantially impede the downstream movement of sediment or water.

Recent advances in stream restoration challenge this view, suggesting that instream obstructions that dynamically alter both the vertical and horizontal position of the stream are essential to the recovery of salmon habitat. There are spatial patterns to the numerous types of instream obstructions that historically existed in watersheds and the ecological functions provided by a particular type of obstruction depends on watershed position. For many types of obstructions, key to creating habitat beneficial to salmon is the ability to substantially decrease the sediment and water transport capacity of a stream, and to do so both stochastically and dynamically. From this perspective, salmon-sustaining streams are recognized as a network of dynamic habitat patches that move in three dimensions.

Sanctuary Forest will quantify expected positive outcomes from this pilot project including increases in summer stream flows, pool habitat, off channel (overwintering) habitat, floodplain connectivity, channel sinuosity, spawning gravels and the extent of floodplain vegetation. The project will be conducted as a scientific research project with development of hypotheses, desired outcomes, potential risks analysis, and pre and post-project monitoring.